Drill bit size converter
Convert between number and letter drill sizes, fractional inches and millimeters, or find the number drill closest to a size.
That's about the same as the tap drill for 8-32 (#29).
What it means
How wide is that hole?
Sizes from ASME B94.11M. Between two listed drills the label shows the nearest one; the millimeters are exact.
Every unit
That drill in every system
Learn
Three things worth knowing
-
Tap drills to remember
#29 for 8-32, #7 for 1/4-20, F for 5/16-18, 5/16" for 3/8-16 and 27/64" for 1/2-13.
-
From wire gauges
Number drills come from the Stubs steel wire gauge of 19th-century England: a #60 drill matches #60 wire. Letters were added later above #1.
-
Numbers run backwards
A #10 drill (4.9 mm) is bigger than a #20 (4.1 mm). Letter drills run the other way: Z is the biggest.
Shortcuts
Popular drill size conversions
Background
About drill sizes
US drill sets come in three series that fill each other’s gaps. Number drills run from #80 (0.35 mm) up to #1 (5.79 mm); the number gets smaller as the drill gets bigger. Letter drills continue from A (5.94 mm) to Z (10.49 mm). Fractional drills go up in 64ths of an inch.
Machinists need all three because tap drills, the holes threaded by a tap, rarely fall on a fraction: a #29 is the hole for an 8-32 screw.